Syrian and Russian air strikes have killed at least 47 people, including 31 civilians, in an ISIL-held town in eastern Syria, a monitor has said.

Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, said that Saturday's raids hit the town of Al Qurayyah in Deir Az Zor province.

At least 31 civilians were identified among those killed, but it was not immediately clear whether the 16 others were civilians or the fighters of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), Abdel Rahman said.

"IS fighters have now set up a security perimeter around the residential area, where the town's mosque is located," said Abdel Rahman, using a different acronym for ISIL.

Russian warplanes have been carrying out an air raids in support of President Bashar al-Assad since September 2015.

Meanwhile, Kurdish and Arab fighters of Syrian Democratic Forces (SDF) overrun a key road junction in the south of Manjib city, an ISIL stronghold, on Saturday after capturing nearby grain silos overnight, the Observatory said.

"The grain silos overlook more than half of Manbij. SDF fighters can climb to the top and monitor the city," said Abdel Rahman.

The Raqqa Revolutionaries Brigades - one of the Arab components of the Kurdish-dominated alliance - confirmed that SDF forces had seized the silos and pushed into the city.

The Mills Roundabout lies less than two kilometres from the city centre.

The Observatory said ISIL and the SDF were locked in intense street fighting as ISIL tried to defend their positions.

Hundreds of Kurds fleeing villages near Manbij controlled by the ISIL group came under fire, amid mass abductions by the group, opposition activists and a Kurdish official said.

One family who fled was struck by a mine that killed two family members and wounded the other three, Sherfan Darwish, an SDF spokesman, told the Associated Press news agency. He said a 10-year-old girl was killed by ISIL sniper fire.

"Civilians are defying death in order to leave areas controlled by Daesh," Darwish said, using an Arabic acronym for ISIL.

The Local Coordination Committees, another activist-run monitoring group, said ISIL also opened fire at people trying to flee from Manbij, killing 10 of them, including children.

The Syria Democratic Council, the political wing of SDF, called on the international community and aid groups to supply those fleeing with whatever they need, saying many of them are in open areas.

The SDC called on the world to help the SDF "prevent the occurrence of a catastrophe or a massacre," saying there were "indications" one might happen.

Strategic battle

Captured by ISIL in 2014, Manbij was a key transit point for foreign fighters and funds, as well as a trafficking hub for oil, antiquities and other plundered goods.

The SDF launched its offensive to take Manbij on May 31, driving across the Euphrates River from the east with military advice from about 200 US special forces troops.

ISIL has thrown large numbers of fighters into the battle, losing 463, according to the Observatory. The SDF has lost at least 89.

Manbij lies in the eastern plains of Aleppo province, which has become a battleground between an array of competing armed groups, including al-Qaeda, moderate rebels and government forces, as well as the SDF and ISIL.